1. Field of Invention
The field of invention is that of radio-communications.
More precisely, the invention relates to the problem of saturation of the radio-communication cells, linked to the 5 number of channels available within the cell and to the maximum transmission power of the base station associated with each cell of the network.
2. Description of the Related Art
It will be recalled that cellular telecommunications networks are constituted by a meshing of their area of coverage into geographic areas of smaller size called cells. These cells themselves are generally sub-divided into geographic sectors. Traditionally, as shown in FIG. 1, a cell 10 is divided into three sectors 111 to 113, each with an angular aperture approximately equal to 120°.
Each cell 10 corresponds to the geographic area covered by a base station 12, which transmits and/or receives signals to and/or from radio-communication terminals 13 present in the cell.
An important problem inherent to radio-communication networks is that the number of communications that can be established, in one cell, between a base station and the various radio-communication terminals present in the cell, is limited. This limitation is linked, on the one hand, to the maximum number of available channels in the cell, and, on the other hand, to the maximum power that the base station can transmit.
According to one known technique, a predetermined number of communication channels is generally attributed to each of the sectors of a cell. A disadvantage of this technique of the prior art is that, as soon as all the channels attributed to a sector are used, any new request for a terminal present in the sector and wishing to establish a communication, is refused.
Consideration has been given to allocating distinct levels of priority to the radio-communication terminals. In this way, when a terminal with a high level of priority transmits a request to open communication, it can have access to channel of communication at the expense of a low priority terminal, which is then compelled to put an end to its communication.
A disadvantage of this technique from the prior art is therefore that within a sector, one can only satisfy a limited number of requests to open communication, even if communication channels are available in other sectors of the cell.
In addition, the number of communications that can be set up in one cell is also limited by the maximum power that the base station can transmit, because of an interference phenomenon.
At present, two sources of interference can be distinguished.
On the one hand, the signals transmitted within a sector, by the radio-communication terminals or by the base station are propagated partially into adjacent sectors, belonging to the cell under consideration, or into a neighboring cell. Within a given sector, interfering signals coming from adjacent sectors are therefore added to the wanted signals in the form of inter-sector interference.
On the other hand, within a given sector, a radio-communication terminal represents a potential source of interference for the neighboring terminals, since the signals intended for a particular terminal are also received by all the neighboring terminals, and thereby constitute inter-sector interference.
It is therefore clearly apparent that, the greater the number of terminals in communication in a cell, the more important are the inter-sector and intra-sector interference phenomena. Indeed, in order to compensate for these interference phenomena and to maintain a good signal to noise ratio for all the communications within a cell, it is necessary to increase the transmission power of the base station of the cell under consideration. When the interference is too great, which is particularly frequent in the case of CDMA (Code Division Multiple Access), the transmission power required at the base station to obtain a signal to noise ratio that is adequate for reception, can become too great and can therefore lead to saturation of the base station.
Therefore, another disadvantage of techniques of the prior art is that, because of an interference phenomenon, the number of communications that can be established within one sector is limited, even if communication channels remain available within the sector.